1. Technical Field
This invention relates to certain polyacetal compositions containing at least one oxo-piperazinyl-triazine hindered amine light stabilizer, said compositions being characterized as having good stability upon exposure to light. Further, and most surprisingly, certain other properties, such as melt processing stability and retention of physical properties upon exposure to heat and light, possessed by a polyacetal polymer are not significantly sacrificed when the oxo-piperazinyl-triazine hindered amine light stabilizer of the present invention is incorporated therein.
Hereinafter, the term "hindered amine light stabilizer" shall be referred to as "HALS". The term "oxo-piperazinyl-triazine" shall be referred to as "PIP-T" for brevity. The triazine ring will usually have three polysubstituted piperazine-2-one ("PSP") substituents, each distally spaced apart from each carbon atom of the triazine ring by a polyalkyleneamine bridge.
Polyacetal, also commonly referred to as polyoxymethylene, compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Compositions based on polyacetals of relatively high molecular weight, i.e., 10,000 to 100,000 are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g. compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping and thermoforming. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance.
In some applications, polyacetal compositions are exposed to light for long periods of time. It is desired that said polyacetal compositions remain relatively stable upon exposure to light for such periods of time. The stability of a polyacetal composition upon exposure to light can be measured by the weight loss it experiences upon exposure to UV light and/or its color fastness. To impart or improve light stability to a polyacetal composition, HALS may be added to the polyacetal compositions. However, it is not uncommon that the addition of such HALS can adversely affect other properties of the polyacetal composition, such as the melt processing stability (e.g., gaseous formation evolution or discoloration during melt processing) of said composition and/or the retention of the physical properties of said composition upon exposure to heat and air.
It has been found, in the present invention, that the inclusion of certain PIP-T HALS into a polyacetal results in a polyacetal composition having good stability upon exposure to light, as measured by weight loss upon exposure to UV light and/or color fastness. Further, it has been found that the inclusion of these certain PIP-T HALS into polyacetal does not significantly adversely affect the other properties, such as the melt processing stability and the retention of physical properties upon exposure to heat and air of the polyacetal composition.
The specific PIP-T HALS useful in the compositions of the present invention are a known compound and more preferably, an alkylated derivative of a compound similar to the known compound.
The known compound is specifically disclosed in U.S. Pat. No. 4,547,538 and European Patent Application 0 299 426 and it has the following structure (I): ##STR1## The nitrogen in the piperazinone ring of the oxo-piperazinyl ring to which the hydrogen (--H) is attached is hereinafter referred to as the hindered N.sup.4 atom. The hindered N.sup.4 atom of (I) is of secondary amine functionality. Structure (I) is hereinafter referred to as the unalkylated secondary amine PIP-T HALS (I).
The preferred PIP-T HALS is a derivative of a compound similar to the known compound (I) and it has the following structure (II): ##STR2## where A=methyl, ethyl, or propyl, preferably methyl; R.sup.1, R.sup.2, R.sup.3, and R.sup.4 each independently represent C.sub.1 -C.sub.6 alkyl, or, when together cyclized, R.sup.1 and R.sup.2, and R.sup.3 with R.sup.4, represent C.sub.5 -C.sub.7 cycloalkyl; R.sup.5 represents C.sub.1 -C.sub.6 alkyl, C.sub.5 -C.sub.7 cycloalkyl, C.sub.7 -C.sub.12 aralkyl, hydrogen and phenyl; R.sup.6 represents C.sub.1 -C.sub.12 alkyl and C.sub.5 -C.sub.12 cycloalkyl; and p represents an integer in the range from 2 to about 12. The nitrogen in the piperazinone ring of the oxo-piperidinyl ring to which the --A substituent is attached in the hindered N.sup.4 atom, as in (I), above. The hindered N.sup.4 atom is alkylated and it is of tertiary amine functionality. Structure (II) is hereinafter referred to as the alkylated tertiary amine PIP-T HALS (II).
Although it is stated in U.S. Pat. No. 4,547,538 that the unalkylated secondary amine PIP-T HALS (I) can be incorporated into any of several organic materials, polymers included, polyacetal is not specifically disclosed as being a material into which this compound can be incorporated. Further, it is not necessarily true that compounds that are effective (or not effective) in most other polymers will also be effective (or not effective) in polyacetal. Part of this reason for this phenomenon is that polyacetal, when melt processed, gives rise to formaldehyde evolution. A compound incorporated into a polyacetal may adversely react with the formaldehyde that is evolved during the melt processing of the polyacetal and thereby may not be effective in the polyacetal even though it is effective in other polymers in which formaldehyde is not evolved during melt processing. Further, the compound may be effective for its particular purpose in polyacetal but, if it reacts with the evolved formaldehyde, it may have a significant adverse effect on the other properties of the polyacetal, thereby rendering it an undesirable compound for inclusion into polyacetal. In contrast, it is also true that a compound not effective in most other polymers may be effective in polyacetal because it reacts in a positive way with the formaldehyde evolved during melt processing. Still another reason for this phenomenon is that polyacetal may be more sensitive to impurities, such as excess acid or base, in the compound and as such, the compound may not be as effective in polyacetal as it would be in other polymers.
As such, even though a compound may be useful in most polymers, if the compound is likely to react adversely with the formaldehyde evolved during the melt processing of the polyacetal and/or if it has an undesirable level of impurities in it, then one skilled in the art of polyacetal chemistry would not expect the compound to be effective in polyacetal. The unalkylated secondary amine PIP-T HALS (I) of the present invention is, at its name implies, a secondary amine. Secondary amines are known to react with formaldehyde, thereby causing discoloration and/or destabilization in polyacetal during melt processing. The effect of secondary amine HALS on polyacetal during melt processing in shown by the Examples herein. However, with the unalkylated secondary amine PIP-T HALS (I) of the present invention, which imparts improved light stability to polyacetal, it was unexpectedly discovered that the discoloration and destabilization experienced by the polyacetal compositions during melt processing is reduced in comparison to the discoloration and destabilization experienced by polyacetal compositions containing other unalkylated secondary amine HALS, provided that the unalkylated secondary amine PIP-T HALS (I) is of proper pH.
Although polyacetal compositions can be effectively stabilized against light degradation (without significantly sacrificing other properties) with the unalkylated secondary amine PIP-T HALS (I), it was surprisingly found that the alkylated tertiary amine PIP-T HALS (II) is even more effective in stabilizing polyacetal compositions against light degradation, without significantly adversely affecting certain other properties, such as melt processing stability and retention of physical properties upon exposure to heat and light, of the polyacetal. It was not expected that polyacetal compositions containing the alkylated tertiary amine PIP-T HALS (II) would possess, on average, better properties than polyacetal compositions containing the unalkylated secondary amine PIP-T HALS (I). Further, it was also unexpectedly found that polyacetal compositions containing the alkylated tertiary amine PIP-T HALS (II) had better properties, on average, than an alkylated tertiary piperidine-s-triazine HALS. As such, even though the unalkylated secondary PIP-T HALS (I) was known and even though alkylated tertiary piperidine-s-triazine HALS were known, the superior results obtained, on average, with the alkylated tertiary amine PIP-T HALS (II) of the present invention were unexpected.
This invention, in summary, is specifically related to polyacetal compositions containing the alkylated tertiary amine PIP-T HALS (II), which is most preferred, or the unalkylated secondary amine PIP-T HALS (I) having the proper pH, said compositions having good stability upon exposure to light in the absence of a significant loss of melt processing stability and retention of physical properties.